The present invention relates to an electro-optical device and a method of controlling the reflection or transmission of an electromagnetic radiation in the electro-optical device. It finds particular application in conjunction with a switchable filter, a laser with tunable lasing, a beam steering device, a wavelength multiplexing device, a telecommunication device, an e-book, a display such as a LCD TV, and an electrically-tunable color display, and will be described with particular reference thereto. However, it is to be appreciated that the present exemplary embodiment is also amenable to other like applications.
Liquid crystal (LC) blue phases (BPs) are among the most interesting self-organized structures in the field of liquid crystals, which have been under numerous studies in the past decades. Liquid crystals generally are anisotropic liquids rich with physical properties; however, blue phase LCs are an exception, as they are optically isotropic liquid and are not birefringent. Although BPs are optically active and rotate in the direction of polarization of linearly polarized light as the helical phase, a small double twist structure is more stable than a single twist structure of a cholesteric LC, as the structure of BP is stabilized by its coexistence with disclination lines.
BPs are a highly fluidic isotropic medium that possesses a three-dimensional periodical structure useful for field-induced birefringence. The field-induced birefringence does not need alignment layers of substrates as those of the conventional liquid crystal displays (LCDs) do. There is however a major limitation for the possible applications of BPLCs because of its narrow phase transition temperature. Recently, the problem has been improved by using a polymer network (see H. Kikuchi, H. Higuchi, Y. Haseba, T. Iwata, Fast Electro-optical switching in polymer-stabilized liquid crystalline blue phases for display application, SID Digest, 37, 1737, 2007) or bimesogenic LC to stabilize the BP. Both methods yield a BP with a temperature that exceeds 50K. It is believed that chiral pitch of the BP affects its phase range, but the mechanism for widening the BP temperature range is still not clear. A large BP temperature range using polymer stabilization led to the recent development of a blue phase mode LCD TV. Lasing in a three-dimensional photonic crystal of the liquid crystal blue phase II was recently reported in W.-Y. Cao, et al., Lasing in a three-dimensional photonic crystal of the liquid crystal blue phase II, Nature Mater. 1, 111-113 (2002).
Liquid crystals are also widely used in optical tuning, which is of value in devices ranging from lasing, laser beam steering, switchable wavelength filters and wavelength multiplexing for telecommunication. For example, liquid crystals can be used for switchable filters in configurations by varying the magnitude of the retardation value of the output polarization. Previous efforts to produce a liquid crystal tunable filter (LCTF) focused on polarization interference filters. Liquid crystal molecules rotate in orientation to the initial optic axis aligned by the surface alignment layers in response to an applied voltage. However, because of the trade-off between throughput and spectral purity, the grating or filter structure is not optimum for implementing a multiple wavelength filter. An example of electrically tunable filter is fabricated by exposing a nematic liquid-crystal and photosensitive pre-polymer mixture to an interference pattern generated by a laser source. The grating structures form switchable and tunable reflection gratings with reflection wavelengths in the ultraviolet, visible and infrared regions, depending on fabrication conditions.
Current transmissive type flat panel displays requires color filters to generate vivid color images. Their light efficiency is low because light passes through polarizers a total of four times. The light efficiency is thus decreased by the absorption of the polarizer (˜6%). For particle-based reflective displays including electrophoretic displays, quick-response-particle displays and electrowetting displays, their brightness is dimmed due to the light absorption of color filters.
Currently, most of the cholesteric liquid crystal based spectral filters and reflective cholesteric displays, whose Bragg-reflected wavelength are static types, are tuned only by adjusting the amount of chiral additive to a nematic material. This approach disadvantageously fixes the spectral wavelength of the device; and at most, one can only turn a pixel in the filter on or off but cannot change its spectral wavelength. One may also use a patterned electrode in a particular layer with different periodicity to transmit or reflect a specific wavelength; however, this reduces the filtering capability as well as the light transmission or reflective brightness of a device. In S.-Y. Lu, L.-C. Chien, A polymer-stabilized single layer color cholesteric liquid crystal display with anisotropic reflection, Appl. Phys. Lett. 91, 131119-1 131119-3 (2007), it has been reported that polymer and cholesteric liquid crystal composite films can be used for electrically tunable reflected color. The phase separated polymer enables tuning of the Bragg reflected wavelength by varying the magnitude of the applied electric field.
It has been reported that application of electric fields across BP materials in an electro-optical cell can induce a small shift in Bragg-reflected wavelength at a low applied voltage, according to H. J. Cole, H. P. Gleeson, Electric Field Induced Phase Transitions and Colour Switching in the Blue Phases of Chiral Nematic Liquid Crystals, Mol. Cryst. Liq. Cryst., 167, 213-225 (1989); and H.-S. Kitzerow, The effect of electric fields on blue phases, Mol. Cryst. Liq. Cryst., 202, 51-58 (1991). The small color switches were observed in both BPI and BPII due to the field-induced phase transition to both cholesteric focal conic and homeotropic nematic states. Both of these electric field induced phenomena are described as functions of the applied voltage and frequency. The field induced color switch was found to have two distinct response times associated with it, one of which is fast (˜100 μs) and another much slower (˜1-10 ms).
The field-induced color (FIC or Bragg reflection) of a blue phase liquid crystal material results from the electrostriction-induced strain in BP phase. The electric field induced phenomena are reported as functions of the applied voltage, pulse width and frequency. (H.-S. Kitzeow, P. P. Crooker, S. L. Kwok, J. Xu, G. Heppke, Dynamics of blue-phase selective reflections in an electric field, Phys. Rev. A, 42, 3442-3448 (1990)). The response time of electrically-controllable color is found to be in the range from a few hundreds of microseconds to a few seconds depending on the driving scheme.
Advantageously, the present invention provides an electro-optical device and a method of controlling the reflection and transmission of an electromagnetic radiation, which exhibit merits such as cost-effectiveness; simpler manufacturability due to the removal of requirements of polarizer, color filter, and sometimes alignment; improved temperature stability; wide color gamut; wide range uniform color switching (˜160 nm); adaptability to single or multi-cell technology; and fast switching, among others.